Refining process for producing low alpha tin

ABSTRACT

A method for purifying tin includes exposing an electrolytic solution comprising tin to an ion exchange resin and depositing electrorefined tin from the electrolytic solution. The deposited electrorefined tin has alpha particle emissions of less than about 0.01 counts/hour/cm 2  immediately after the deposition step, and an alpha emissivity of less than about 0.01 counts/hour/cm 2  at least 90 days after the deposition step.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to U.S. Provisional Application No.61/714,059, filed Oct. 15, 2012, U.S. Provisional Application No.61/670,960, filed Jul. 12, 2012, and U.S. Provisional Application No.61/661,863, filed Jun. 20, 2012, each of which are herein incorporatedby reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to high purity tin with reduced alphaparticle emissions for the manufacture of semiconductor equipment or thelike and manufacturing methods for producing such high purity tin.

DESCRIPTION OF RELATED ART

Solders are commonly utilized in semiconductor device packaging and manyother electronic applications. While conventional solders have beenmanufactured primarily from lead, more recent lead-free solders utilizetin and other metals as principal components.

One challenge with respect to the use of tin solders in electronicpackaging applications is that the elemental tin materials used tomanufacture solders contain varying levels of alpha particle emittingisotopes (also referred to as alpha particle emitters). Alpha particleemissions (also referred to as alpha flux) can cause damage to packagedelectronic devices, and more particularly, can cause soft error upsetsand even device failure in certain cases. This concern is compounded asdevice sizes are reduced and alpha emitting solder materials are closerto sensitive locations.

Uranium and thorium are well known as principal radioactive elementsoften present in metallic containing solders, such as tin solders, whichmay radioactively decay according to known decay chains to form alphaparticle emitting isotopes. Of particular concern in tin materials isthe presence of polonium-210 (²¹⁰Po), which is considered to be theprimary alpha particle emitter responsible for soft error upsets.Lead-210 (²¹⁰ Pb) is a decay daughter of uranium-238 (²³⁸U), has ahalf-life of 22.3 years, and β-decays to bismuth-210 (²¹⁰ Bi). However,due to the very short 5.01 day half-life of ²¹⁰Bi, such isotope isessentially a transient intermediary which rapidly decays to ²¹⁰Po. The²¹⁰Po has a 138.4 day half-life and decays to the stable lead-206(²⁰⁶Pb) by emission of a 5.304 MeV alpha particle. It is the latter stepof the ²¹⁰Pb decay chain, namely, the decay of ²¹⁰Po to ²⁰⁶Pb withrelease of an alpha particle that is of most concern in metallicmaterials used in electronic device applications.

Although ²¹⁰Po and/or ²¹⁰Pb may be at least in part removed by meltingand/or refining techniques, such isotopes may remain as impurities in atin material even after melting or refining. Removal of ²¹⁰Po from a tinmaterial results in a temporary decrease in alpha particle emissionsfrom the material. However, it has been observed that alpha particleemissions, though initially lowered, will typically increase over timeto potentially unacceptable levels as the secular equilibrium of the²¹⁰Pb decay profile is gradually restored based on any ²¹⁰Pb remainingin the metallic material.

Problematically, whether an increase in alpha particle emissions of ametallic material following a melting or refining process willeventually reach unacceptable levels is very difficult to assess and/orpredict.

SUMMARY OF THE INVENTION

A method for purifying tin includes exposing an electrolytic solutioncomprising tin to an ion exchange resin and depositing electrorefinedtin from the electrolytic solution. The electrorefined tin can havealpha particle emissions of less than about 0.01 counts/hour/cm² or lessthan about 0.002 counts/hour/cm². The ion exchange resin may includesulfonated, phosphomethylated, amino methyl phosphonic acid, andpoly(4-vinyl-pyridine) functional groups and combinations of thesefunctional groups. The electrolytic solution may have a pH of less thanabout 6 or about 1 or less.

The method for purifying tin may further include assessing the alphaparticle emission potential of the electrorefined tin, includingdetecting alpha particle emissions from a sample of the depositedelectrorefined tin, determining a concentration of a target parentisotope in the sample from the alpha particle emissions detected in thedetecting step and a time which has elapsed between the detecting stepand the exposing and detecting steps, and determining a possible alphaemission of a target decay isotope of the target parent isotope from thedetermined concentration of the target parent isotope and the half-lifeof the target parent isotope.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of an electrorefining system.

FIG. 2 is a plot of alpha particle emissions over time forelectrorefined tin samples.

DETAILED DESCRIPTION I. Method of Electrorefining Tin

As described herein, tin may be electrorefined to produce refined tinhaving reduced alpha particle emissions or alpha flux when measuredafter the electrorefining process. The alpha particle emissions do notnecessarily remain stable after the material has been subjected to anelectrorefining process, and the alpha particle emissions may increaseor decrease over time. As described herein, the refined tin may alsohave reduced alpha particle emissions when measured a period of timefollowing the electrorefining process, such as 90 days after theelectrorefining process. A method for determining the alpha particleemission potential, such as the maximum alpha particle emissions, for arefined tin is also described herein.

Tin may be electrorefined by depositing tin ions from an electrolyticsolution onto a cathode by applying a current to the system. Anelectrolytic solution containing tin or stannous ions may be formed bydissolving or leaching tin in an acid electrolyte. For example, tinsulfate can be formed by an electrolytic dissolution of a 99.99% puritytin anode in an electrolyte including 1% to 10% sulfuric acid by volumemixed with deionized water. Suitable concentrations of soluble stannousion in the electrolytic solution include but are not limited to fromabout 10 g/L to about 200 g/L. More particularly, suitableconcentrations of soluble stannous ion in the electrolytic solution maybe as low as 10, 20, 30, 40, 50, 60 g/L or as great as 80, 100, 120,140, 160, 180 or 200 g/L or may be within any range delimited by anypair of the foregoing values. At low tin concentrations, such as 40, 30,20 g/L or less, the alpha particle emissions of the deposited materialmay be more sensitive to the current density of the electrorefiningprocess than at higher tin concentrations

In certain embodiments, the electrolytic solution may be formed byadding a commercially available tin, such as commercially available tinhaving a purity level of 99.0% to 99.999% (2N to 5N), to the acidicelectrolyte. In one example, the tin may have initial, pre-refiningalpha particle emissions above about 0.001 counts/hour/cm². In otherexamples, the tin may have initial, pre-refining alpha particleemissions above about 0.002 counts/hour/cm², above about 0.005counts/hour/cm², or above about 0.01 counts/hour/cm².

The electrolytic solution may include one or more acids. Suitable acidsfor use in the acidic electrolytic solution include but are not limitedto hydrochloric acid, sulfuric acid, fluoroboric acid, acetic acid,methane sulfonic acid, and sulfamic acid. The acid may be mixed withwater, such as deionized water. The acid(s) of the electrolytic solutioncan be selected to control the pH of the electrolytic solution.

The electrolytic solution may have a low, or acidic, pH. For example, anelectrolytic solution having an acidic pH may have a pH of less than 7.In another example the electrolytic solution may have a pH of less thanabout 6. In a further example, the electrolytic solution may have a pHof less than about 5. In a still further example, the electrolyticsolution may have a pH of less than about 4, less than about 3, lessthan about 2 or less than about 1. The pH of the electrolytic solutionmay be adjusted to optimize the effectiveness of the ion exchange resinand the electrorefining process.

The electrolytic solution may optionally include one or more additives.As used herein, an “additive” refers to a component of the electrolyticsolution other than the target metal to be refined (e.g., tin), othermetallic impurity components, and the acid/water solution. The additivemay be helpful for controlling one or more properties of theelectrolytic solution, the deposition process and/or the depositedproduct. Each additive may be present in amount from severalparts-per-million (ppm) to several percent by weight. For example, eachadditive may be present in an amount of at least about 0.05% by volumeof the electrolytic solution, at least about 0.5% by volume of theelectrolytic solution, or at least about 1.0% by volume of theelectrolytic solution.

Suitable additives include antioxidants and grain refiners. For example,an antioxidant may be added to the electrolytic solution to preventspontaneous Sn²⁺ to Sn⁴⁺ oxidation during electrolysis. Suitableantioxidants include, but are not limited to, phenol sulfonic acid andhydroquinone. Suitable commercially available antioxidants includeTechnistan Antioxidant, Techni Antioxidant Number 8 available fromTechnic, and Solderon BP Antioxidant available from Dow Chemical.Suitable concentrations of an antioxidant include from about 0.05% toabout 10%, from about 0.5% to about 5%, or from about 1% to about 3% byvolume of the electrolytic solution.

An organic grain refiner may optionally be added to the electrolyticsolution to limit dendritic deposition at the cathode. Suitable organicgrain refiners include, but are not limited to, polyethylene glycol.Suitable commercially available organic grain refiners includeTechnistan TP-5000 Additive, Techni Matte 89-TI available from Technic,and Solderon BP Primary available from Dow Chemical. Suitableconcentrations of a grain refiner include from about 0.5% to about 20%,from about 1.0% to about 15%, or from about 3% to about 10% by volume ofthe electrolytic solution.

The electrolytic solution is exposed to at least one ion exchange resinduring at least a portion of the electrorefining process. Ion exchangeresins are organic compounds which include functional groups configuredto selectively capture another material by exchanging ions with thecaptured material. For example, ion exchange resins may includefunctional groups bonded to a polymer matrix. In the current process, itis believed that the ion exchange resin captures and removes alphaemitting impurities from the electrolytic solution, such as metallicimpurities and, in particular, metallic impurities which are eitherthemselves capable of decay with concurrent release of an alphaparticle, such as ²¹⁰Po, or metallic impurities which produce decayproducts with the decay products capable to decay with concurrentrelease of an alpha particle, such as U and/or Th.

In one example, the ion exchange resin may be placed in a column and theelectrolytic solution may be circulated through the column. For example,the electrolytic solution may be circulated from a tank, through the ionexchange resin column and returned to the tank by a pump. In thisembodiment, the electrolytic solution may be circulated through thecolumn of ion exchange resin concurrently with application of current tothe electrolytic bath, or alternatively, the circulation of theelectrolytic solution through the ion exchange resin may occur prior to,or after, application of current according to a desired quantify and/orduration. In a still further embodiment, circulation of the electrolyticsolution through the ion exchange resin and application of current maybe alternated as desired. The flow rate through the column may beadjusted to achieve a desired contact time between the electrolyticsolution and the ion exchange resin. In an alternative embodiment, theresin may be added directly to the tank holding the electrolyticsolution; a separate column is not used.

Suitable ion exchange resins may include at least functionalizedcarboxylic acid from the phosphonic acids group, such as amino methylphosphonic acid functional groups. Further suitable ion exchange resinsmay include at least one functional group selected from sulfonated,phosphomethylated, amino methyl phosphonic acid, andpoly(4-vinyl-pyridine) functional groups and mixtures thereof. Stillfurther suitable ion exchange resins may include at least one functionalgroup selected from sulfonated, phosphomethylated, amino methylphosphonic acid, poly(4-vinyl-pyridine), sulfonic acid, chloromethyl,tributylamine, di-vinyl benzene, quaternary amine, divinylbenzene,diphosphonic acid, and iminodiacetate functional groups. Examples ofcommercially available suitable ion exchange resins are presented inTable 1, where “DVB” is divinylbenzene, “SB” is strong base, “SA” isstrong acid, “WA” is weak acid, and “Dow” is Dow Chemical Company.

TABLE 1 Data for Select Ion Exchange Resins Functional Exchange Tradename Vendor Group(s) mechanism Matrix Monophos Resin Eichrom Sulfonatedand Chelating Styrene-DVB phosphomethylated Lewatit MonoPlus LanxessAmino methyl Cation Crosslinked TP 260 phosphonic acid exchangepolystyrene Reillex HPQ Vertellus Poly(4-vinyl- Anion DVB Polymerpyridine) exchange Dowex G-26 Dow Sulfonic acid Cation Styrene-DVB,exchange gel Dowex Optipore Dow Chloromethyl Adsorbent Styrene-DVB, L493Macroporous Dowex PSR-2 Dow Quatenary amine Anion Styrene-DVB, gelexchange Dowex 21K XLT Dow Quatenary amine SB anion Styrene-DVB,exchange gel Dowex MAC-3 Dow Carboxylic acid WA cation Polyacrylic,exchange macroporous XZ 91419.00 Dow Quatenary amine SB anionStyrene-DVB, resin exchange gel XUS 43568 resin Dow Di-methyl amine WBanion Macro Styrene exchange Amberlyst A-26 Dow Quaternary amine AnionStyrene-DVB exchange Amberlyst 15WET Dow Sulfonic acid SA cationStyrene-DVB, exchange macroporous Amberlite IRC- Rohm Amino-phosphate;Chelating Styrene-DVB, 747 and Haas Na+ form macroporous Amberlite PWA 5Rohm SB anion, NO₃ ⁻ Anion Cross-linked and Haas selective exchangecopolymer Diphonix resin Eichrom Diphosphonic acid ChelatingStyrene-based and sulfonic acid polymer Lewatit TP 207 LanxessIminodiacetate Cation Crosslinked exchange polystyreneAn ion exchange resin may be used alone or in combination with other ionexchange resins. In particular, a mixed bed resin may be used, where amixed bed resin refers to a resin composition that includes two or morespecific resins that may have the same or different functional groups,exchange mechanisms and/or matrices.

Tin from the electrolytic solution is plated onto a cathode during theelectrorefining process. In some embodiments, exposing the electrolyticsolution to the ion exchange resin and electrodeposition of the tin ontothe cathode may occur at least partially concurrently. As describedfurther below, the electrorefined tin may have reduced alpha particleemissions or alpha flux.

FIG. 1 is a block diagram illustrating an exemplary continuous tinelectrorefining system 100 including tank 110, cathode 112, first tinanode 114A and second tin anode 114B (collectively referred to as tinanodes 114), media column 116, pump 118, filter 120, pump 122, andrectifier 124, which is capable of generating the required currentdensity. One or more cathodes 112 and one or more tin anodes 114 arepositioned in tank 110. As shown in FIG. 1, tin anodes 114 may be placedon either side of cathode 112. Tank 110 also contains an electrolyticsolution containing tin, which has been described above. Theelectrolytic solution is circulated through media column 116 by pump 118and is returned back to tank 110. Media column 116 contains an ionexchange resin. The flow rate through media column 116 is calculated toachieve a specified contact time between the electrolytic solution andthe ion exchange resin. Adjusting the flow rate through media column 116may vary the contact time. For example, increasing the flow rate of theelectrolytic solution through media column 116 may decrease the contacttime between the electrolytic solution and the ion exchange resin andconversely, decreasing the flow rate of the electrolytic solutionthrough media column 116 may increase the contact time between theelectrolytic solution and the ion exchange resin.

System 100 may also include filter 120. The electrolytic solution fromtank 110 may be pumped through filter 120 by pump 122 and returned backto tank 110. Filter 120 may filter particulate matter from the solution.For example, filter 120 may remove material have a size greater thanabout 5 microns.

Rectifier 124 is connected to cathode 112 and anodes 114 and providesthe required current density for dissolution of tin anodes 114 andelectrodepositing tin from the electrolytic solution onto cathode 112during the electrodepositing or electrorefining process. A suitablecurrent density at the cathode may be as low as 10, 15, 20, 25, 30, 35,40 amps per square foot (ASF) or as great as 25, 30, 35, 40, 45, 50, 55,60, 65 or 70 ASF or may be within any range delimited by any pair of theforegoing values. In other embodiments, the current density at thecathode may be as low as 70, 80, 90, 100, 125 or 150 ASF or as great as175, 200, 225, 250, 275 or 300 ASF or may be within any range delimitedby any pair of the foregoing values. In one example, the current densitywas regulated at about 22 milliamps per square centimeter (mA/cm²) (20ASF) at cathode 112 and about 8-11 mA/cm² (7-10 ASF) at anodes 114.

The tin may be refined in a continuous process as described above. Forexample, the steps of exposing the electrolytic solution to an ionexchange resin and depositing the tin from the electrolytic solutiononto a cathode may occur at least partially concurrently.

Alternatively, the tin may be refined in a step or batch process. Forexample, an electrolytic solution may be formed by electrolyticdissolution of tin anodes and a permeable membrane may be used toprevent tin from depositing on the cathode. The dissolution may then bestopped, and the electrolytic solution may be exposed to an ion exchangeresin for a period of time. For example, the electrolytic solution maybe passed through a column containing the ion exchange resin or the ionexchange resin may be added to the electrolytic solution tank. Afterexposure to the ion exchange resin, the electrolytic solution may beelectrodeposited onto a cathode.

In some embodiments, the eletrorefining system may include two or moreelectrodeposition processes. Each electrodeposition process may includethe same or different electrolytic solution compositions. For example,the electrolytic solutions may include the same or different acidsand/or additive(s) and/or have the same or different pH. One or more ofthe electrodeposition processes may including an ion exchange resin asdescribed herein, and if present in two or more of the processes, theion exchange resin may be the same or different. In some embodiments,two or more electrodeposition processes may be conducted in series or insuccession such that tin ions are electrodeposited two or more times.For example, the electrorefining system may include electrodepositingtin ions from an electrolytic solution containing hydrochloric acid ontoa cathode, electrolytic dissolution of the deposited tin into a secondelectrolytic solution containing sulfuric acid, and electrodepositingtin ions from the second electrolytic solution onto a second cathode.Impurities and/or contaminant components may be removed in eachsuccessive electrodeposition process. Further, different impuritiesand/or contaminant components may be removed based on the electrolyticsolution composition and/or the ion exchange resin of theelectrodeposition process.

In some embodiments, the electrorefined tin may not experience asignificant reduction in lead content compared to that of the tin priorto the electrorefining process (e.g., the input or pre-refined tin). Forexample, the lead content may not be reduced by more than about 1% andparticularly not by more than about 0.1% by the electrorefining process.A suitable lead content of the tin prior to the electrorefining processmay be at least 1 ppm and more particularly at least about 2 ppm. Asuitable lead content of the electrorefined tin may be at least about 1ppm and more particularly at least about 2 ppm. In some embodiments, thelead content of the electrorefined tin may be as low as 0.01, 0.05 or1.0 ppm or as great as 2.0, 5.0 or 10.0 ppm or may be within any rangedelimited by any pair of the foregoing values.

It has been found that electrodeposited tin which is produced byexposing the electrolytic solution to at least one ion exchange resinduring electrorefining has reduced alpha particle emissions or alphaflux.

Although there is a relationship between a reduction in certainimpurities such as thorium and a reduction in alpha particle emissions,a tin material having less than 1 ppm thorium will not necessarily havea sufficient low alpha particle emissions or alpha flux to satisfycertain industry requirements. For example it is entirely possible torefine tin to a 6N purity level without reducing alpha particleemissions to a suitable level. Accordingly, in one example,electrorefined tin may be tested for alpha particle emissions afterrefining using, for example, a gas flow proportional counter such as anAlpha Sciences 1950 in the manner described in JEDEC standard JESD221.

The overall reduction in alpha particle emissions will vary depending onmany factors including, but not limited to, the alpha particle emissionsof the input or pre-refined tin material, the contact time of theelectrolytic solution with the ion exchange resin, and the number ofpasses of the electrolytic solution through the ion exchange resin. Inone example, the alpha particle emissions of the refined tin material isreduced by at least 50%, more particularly at least 75%, and even moreparticularly at least 85%, 90% or 95% compared to the alpha particleemissions of the same material prior to deposition of the electrorefinedtin. In another example, electrorefining is carried out under conditionssuitable to reduce the alpha particle emissions of the refined tinmaterial to less than about 0.01 counts/hour/cm², more particularly lessthan about 0.002 counts/hour/cm², and even more particularly less thanabout 0.001 counts/hour/cm².

It should be noted that the alpha particle emissions of tin does notnecessarily remain stable after the material has been refined. Inparticular, alpha particle emissions or alpha flux of the refined tinmay increase or decrease over time due to the residual presence andradioactive decay of various elements such as ²¹⁰Pb. The increase ordecrease of alpha particle emissions over time may be referred to asalpha drift.

As described herein, it has surprisingly been found that not only doesthe electrorefining process including an ion exchange resin reduce thealpha particle emissions of the electrorefined tin immediately after theelectrorefining process but it also results in reduced alpha drift andreduces the alpha particle emissions at a period of time after theelectrorefining process. In one embodiment, the alpha particle emissionsof the refined tin 90 days after the electrorefining process is at least50%, more particularly at least 75%, and even more particularly at least85%, 90% or 95% less than the alpha particle emissions of the samematerial prior to electrorefining. In another example, theelectrorefining is carried out under conditions suitable to reduce thealpha particle emissions of the electrorefined tin to less than about0.01 counts/hour/cm², more particularly less than about 0.002counts/hour/cm² and even more particularly less than about 0.001counts/hour/cm², when measured 90 days after the electrorefiningprocess.

II. Method of Determining the Alpha Particle Emission Potential ofElectrorefined Tin

A method for determining the alpha particle emission potential of theelectrorefined tin, such as the maximum alpha particle emissions fromthe tin, is described herein. The described method, for example, can beused to predict or forecast the maximum alpha particle emissions fromthe tin.

As used herein, the term “target parent isotope” refers to an isotope ofinterest which is present in a metallic material and is able to decay toa daughter isotope, wherein the daughter isotope may subsequentlyalpha-decay, i.e., may decay to a further isotope with concomitantemission of an alpha particle. The term “target decay isotope”, as usedherein, refers to an isotope of interest which is a daughter isotope ofthe target parent isotope and itself may subsequently alpha-decay, i.e.,may decay to a further isotope with concomitant emission of an alphaparticle. The target decay isotope may or may not be itself a directdecay product of the target parent isotope. For example, if ²¹⁰Pb is atarget parent isotope, ²¹⁰Po may be a target decay isotope even though²¹⁰Pb decays to ²¹⁰Bi with subsequent decay of ²¹⁰Bi to ²¹⁰Po.

According to the present method, the metallic material (e.g., tin) issubjected to a secular equilibrium disruption process. As used herein,the term “secular equilibrium disruption process” refers to a process towhich the metallic material is subjected which at least partiallydisrupts the secular equilibrium of the decay profile of at least onetarget parent isotope within the metallic material. In most instances,the secular equilibrium disruption process disrupts the secularequilibrium of the decay profile of a target parent isotope by reducingthe concentration of the target parent isotope in the metallic material,by reducing the concentration of a corresponding target decay isotope inthe metallic material, or by a combination of the foregoing. Theelectrorefining process described herein is an exemplary secularequilibrium disruption process. Other exemplary secular equilibriumdisruption processes include melting, casting, smelting, refining (suchas electro-chemical refining, chemical refining, zone refining, andvacuum distillation). A secular equilibrium disruption process may alsoinclude any combination of two or more of the foregoing processes.Typically, in the secular equilibrium disruption process, andparticularly when the secular equilibrium disruption process is at leastin part a refining process, both the target parent isotopes and thetarget decay isotopes are at least partially removed as impurities byphysical and/or chemical separation from the bulk metallic material.

In some embodiments, the secular equilibrium disruption process mayremove substantially all of a given target decay isotope and therebyeffectively “reset” the secular equilibrium of the corresponding targetparent isotope. For example, in the case of a metallic materialincluding ²¹⁰Pb as a target parent isotope, the secular equilibriumdisruption process may substantially completely remove all of the ²¹⁰Potarget decay isotope in the material, such that the secular equilibriumof ²¹⁰Pb is effectively reset, wherein substantially all ²¹⁰Po that ispresent in the material following the secular equilibrium disruptionprocess is generated by decay of ²¹⁰Pb after the said disruptionprocess. However, the present process may also be practiced usingsecular equilibrium disruption processes that remove only a portion ofthe target parent isotope and/or target decay isotope, and the presentprocess is not limited to secular equilibrium disruption processes thatremove substantially all of a given target decay isotope.

In some embodiments, the secular equilibrium disruption process may becompleted in a relatively short amount of time and, in otherembodiments, the secular equilibrium disruption processes may require arelatively greater amount of time for completion, depending on thenature of the process and the number of processes that together mayconstitute the secular equilibrium disruption process. Therefore, theelapsed time discussed below, between the secular equilibrium disruptionprocess and the measurement of alpha particle emissions of the metallicmaterial, may be an elapsed time between the completion of the secularequilibrium disruption process (or processes) and the measurement ofalpha particle emissions of the metallic material.

After the metallic material (e.g., tin) is subjected to the secularequilibrium disruption process, the alpha particle emission of themetallic material is detected, i.e., an alpha particle emissionmeasurement is obtained. Although it is within the scope of the presentdisclosure to obtain an alpha particle emission of the entire metallicmaterial in bulk form, typically a sample of the bulk metallic materialwill be obtained for purposes of alpha particle emission analysis.

A relatively thin portion of the bulk metallic material may be obtainedas a sample by a suitable method such as rolling the bulk metallicmaterial to provide a thin sheet of sample material, or by any otheranother suitable method.

After the sample is obtained, the sample is treated by heat in order topromote diffusion of target decay isotopes in the sample material untilsuch point that the concentration of atoms of the target decay isotopesin the sample is uniform throughout the sample volume. In many samples,there may be a larger concentration of atoms of target decay isotopestoward the center of the sample, for example, or otherwise in otherareas of the sample such that a concentration mismatch or gradient ispresent. The heat treatment removes any such concentration mismatches orgradients by promoting diffusion of atoms of target decay isotopeswithin the sample from areas of relatively higher concentration towardareas of relatively lower concentration such that a uniformconcentration of target decay isotopes is obtained within the sample.When such uniform concentration is obtained, the number of atoms oftarget decay isotopes within a detection limit depth of the alphaparticle detection process will be representative of and, moreparticularly will correlate directly to, the uniform concentration ofatoms of target decay isotopes in the entirety of the sample. Suchuniform concentration is achieved when the chemical potential gradientof the target decay isotopes is substantially zero and the concentrationof the target decay isotopes is substantially uniform throughout thesample.

Stated in another way, at room temperature, the test sample may have achemical potential gradient, in that the concentration of target decayisotopes is higher on one side of the sample than another side of thesample, or at the centroid of the sample than at the outer surfaces ofthe sample. Heating of the sample adjusts the chemical potentialgradient and, at a sufficient time and temperature exposure, thechemical potential gradient is substantially zero and the concentrationof the target decay isotopes is substantially uniform throughout thesample.

As used herein, the term “detection limit depth” refers to a distancewithin a given metallic material through which an emitted alpha particlemay penetrate in order to reach a surface of the material and thereby bereleased from the material for analytical detection. Detection limitdepths for ²¹⁰Po in selected metallic materials are provided in Table 2below, in microns, which is based on the penetration of the 5.304 MeValpha particle released upon decay of ²¹⁰Po to ²⁰⁶Pb:

TABLE 2 Detection limit depths of ²¹⁰Po in selected metallic materialsDetection limit depth of Metallic material ²¹⁰Po (microns) Tin (Sn) 16.5Aluminum (Al) 23.3 Copper (Cu) 11 Bismuth (Bi) 17.1

The detection limit depth for alpha particles of differing energy, suchas alpha particles emitted upon radioactive decay of alphaparticle-emitting isotopes other than ²¹⁰Po, will vary, with thedetection limit depth generally proportional to the energy of the alphaparticle. In the present method, emitted alpha particles may be detectedby use of a gas flow counter such as an XIA 1800-UltraLo gas ionizationchamber available from XIA L.L.C. of Hayward, Calif. according themethod described by JEDEC standard JESD 221.

Target decay isotopes such as ²¹⁰Po are known to diffuse or migratewithin metallic materials and, in this respect, the heat treatment ofthe present method is used to promote diffusion of the target decayisotope within the material sample to eliminate concentration gradients.In particular, target decay isotopes, such as ²¹⁰Po, will have adiffusion rate J in a given metallic material, which can be expressedaccording to equation (1) below:

$\begin{matrix}{J = {{- D}\frac{\partial\varphi_{P\; o}}{\partial x}}} & (1)\end{matrix}$

wherein:∂φ/∂x is the concentration gradient of the target decay isotope, such as²¹⁰Po; and D is the diffusion coefficient.

The concentration gradient of the target decay isotope is determined bymeasuring the alpha particle emissions at the surface of a sample,removing a layer of material of x thickness, such as by chemicaletching, and measuring the alpha particle emissions at the x depth. Theconcentration of the target decay isotope at the original surface and atdepth x is directly proportional to the alpha particle emission at eachsurface, and concentration gradient of the target decay isotope iscalculated as the difference between the concentration at one of thesurfaces and the concentration at depth x over the distance x.

To determine the polonium diffusion rate J, the polonium alpha particleemissions from 5-5.5 MeV in a tin sample was measured. The sample wasthen heated at 200° C. for 6 hours, and the alpha particle emissionmeasurement was repeated. The number of polonium atoms N is calculatedfrom equation (2) below:

N=A/λ _(Po)  (2)

wherein:A is the alpha particle emission measured in counts/hr; andλ_(Po)=ln 2/138.4 days, based on the half-life of ²¹⁰Po.

The number of moles of polonium calculated by dividing the number ofpolonium atoms N by Avogadro's number. Dividing the difference in thenumber of moles of polonium by the sample area (0.1800 m²) and the timeover which the sample was heated (6 hours) yields a lower bound on thediffusion rate of 4.5×10⁻²³ mol·m⁻²·s⁻¹ at 473K in tin.

TABLE 3 Data for diffusion rate determination Measurement A (Counts/Hr)N (atoms) Moles Initial 24.75 1.19E+05 1.97E−19 Final 46.71 2.24E+053.72E−19

Based on equation (1), one may determine a suitable time and temperatureheating profile to which the sample may be exposed in order to diffusethe target decay isotope within the sample sufficiently to eliminate anyconcentration gradients, such that detection of alpha particle emissionswithin the detection limit depth of the sample is representative, anddirectly correlates, to the concentration of the target decay isotopethroughout the sample. For example, for a tin sample having a thicknessof 1 millimeter, a heat treatment of 200° C. for 6 hours will ensurethat any concentration gradients of ²¹⁰Po atoms within the sample areeliminated.

Thus, for a given metallic material and sample size, the application ofheat may be selected and controlled by time and temperature exposure ofthe sample to ensure that atoms of a target decay isotope are diffusedto a sufficient extent to eliminate concentration gradients. It has beenfound that, by the present method, in providing a suitable time andtemperature profile for the heat treatment step, measurement of alphaparticle emissions from a target decay isotope present within thedetection limit depth directly corresponds to the concentration ornumber of target decay isotope atoms within the entirety of the sample.

It is generally known that subjecting a metallic material to heatpromotes diffusion of elements within the material. However, priormethods have employed heat treatment simply to increase the number ofalpha particle emissions detected over background levels to therebyincrease the signal to noise ratio of the alpha particle emissiondetection.

The alpha particle emissions attributable to ²¹⁰Po is expressed aspolonium alpha activity, A_(Po), at a time (t) following the secularequilibrium disruption process. From the A_(Po) and elapsed time (t),the concentration of ²¹⁰Pb atoms in the sample can be calculated usingequation (3):

$\begin{matrix}{\lbrack {\,^{210}{Pb}} \rbrack_{0} = {\frac{\lambda_{Po} - \lambda_{Pb}}{\lambda_{Po}{\lambda_{Pb}( {^{{- \lambda_{Pb}}t} - ^{{- \lambda_{Po}}t}} )}}( {{A_{Po}(t)} + {{A_{Po}( t_{0} )}^{{- \lambda_{Po}}t}}} )}} & (3)\end{matrix}$

wherein:λ_(Po)=ln 2/138.4 days, based on the half-life of ²¹⁰Po;λ_(Pb)=ln 2/22.3 years (8,145.25 days) based on the half-life of ²¹⁰Pb;andtime (t) is the time which has elapsed between the secular equilibriumdisruption process and the alpha particle emission measurement.

Due to the fact that ²¹⁰Pb has a 22.3 year half-life, the ²¹⁰Pbconcentration is substantially constant over the time (t), particularlywhen the time (t) is less than three years. Also, when substantially allof the ²¹⁰Po is removed in the secular equilibrium disruption process(which may be the case when the secular equilibrium disruption processis a strenuous refining process, for example) the last term in equation(3) above is very near to zero because the initial ²¹⁰Po concentrationwill be very near to zero when the alpha particle emissions are measuredrelatively soon after the secular equilibrium disruption.

The concentration of the target parent isotope may be calculated by theabove-equation (3) and, once the concentration of the target parentisotope is calculated, the known half-life of the target parent isotopemay be used to provide an assessment or prediction of a maximumconcentration of the target decay isotope within the material based onthe re-establishment of the secular equilibrium profile of the targetparent isotope.

In other words, once the concentration of ²¹⁰Pb atoms is determinedusing equation (3), based on the half-life of ²¹⁰Pb the maximum ²¹⁰Poactivity at re-establishment of secular equilibrium will occur at(t)=828 days, and is calculated from equation (4) below:

$\begin{matrix}{{A_{Po}( {t = {828d}} )} = {{\frac{\lambda_{Pb}\lambda_{Po}}{\lambda_{Po} - \lambda_{Pb}}\lbrack {\,^{210}{Pb}} \rbrack}_{0}( {^{{- \lambda_{Pb}}828d} - ^{{- \lambda_{Po}}828d}} )}} & (4)\end{matrix}$

Consistent time units (i.e., days or years) should be used acrossequation (3) and equation (4).

The maximum ²¹⁰Po activity directly correlates to a maximum alphaparticle emission of the material, and will occur at 828 days from thesecular equilibrium disruption process. In this manner, due to the factthat the present method will typically be carried out relatively soonafter the secular equilibrium disruption process, the calculated maximumconcentration of the target decay isotope and concomitant alpha particleemission will typically be a maximum future concentration of the targetdecay isotope and concomitant alpha particle emission that the metallicmaterial will exhibit over a timeframe which corresponds to thehalf-life of the target parent isotope.

For example, based on the half-life of ²¹⁰Pb, the applicable timeframeor “window” by which a maximum possible concentration of ²¹⁰Po (andthereby a peak in alpha particle emissions) will be reached in thematerial will occur at 828 days (27 months) from the secular equilibriumdisruption process.

It is also possible to calculate a possible concentration of ²¹⁰Po (andthereby the alpha particle emissions) at any specified elapsed time fromthe secular equilibrium disruption process. In this manner, it ispossible to calculate a possible concentration of ²¹⁰Po after asufficient elapsed time from the secular equilibrium disruption process,where the sufficient elapsed time may be at least 200, 250, 300, 350 or365 days from the secular equilibrium disruption process. For example,based on the half-life of ²¹⁰Pb, the applicable timeframe by which the²¹⁰Po concentration will reach 67% of the maximum possible concentrationin the material will occur at 200 days from the secular equilibriumdisruption process. Similarly, the ²¹⁰Po concentration will reach 80%and 88% of the maximum possible concentration in the material at 300days and 365 days, respectively, from the secular equilibrium disruptionprocess.

Advantageously, according to the present method, after a metallicmaterial has been subjected to a secular equilibrium disruption processsuch as by refining the metallic material, a maximum alpha particleemission that the metallic material will reach during the useful life ofthe material may be accurately predicted. In this manner, the presentmethod provides a valuable prediction of the maximum alpha particleemission for metallic materials, such as solders, that are incorporatedinto electronic devices.

III. Examples

The present invention is more particularly described in the followingexamples that are intended as illustration only, since numerousmodifications and variations within the scope of the present inventionwill be apparent to those skilled in the art. Unless otherwise noted allparts, percentages and ratios reported in the following examples are ona volume basis, and all reagents used in examples were obtained, or areavailable, from the chemicals suppliers described below, or may besynthesized by conventional techniques.

Example 1 Inclusion of Ion Exchange Resin in an Electrorefining ProcessMaterials Used

Monophos resin: an ion exchange resin having sulfonated andphosphomethylated functional groups and available from Eichrom.

Lewatit MonoPlus TP 260: an ion exchange resin having amino methylphosphonic acid functional groups and available from Lanxess.

Reillex HPQ Polymer: an ion exchange resin having poly(4-vinyl-pyridine)functional groups and available from Vertellus.

Electrorefining Process

An electrolytic solution was added to a 30 liter (L) polypropylene tankequipped with a vertical pump for solution agitation and filtration. Acentral titanium cathode and two 4N tin anodes (one on each side of thecathode) were positioned in the tank, and a DC power supply wasconnected to the cathode and anodes for generating the required currentdensity. During the electrorefining process, the DC current passingbetween the cathode and anodes was regulated to 22 mA/cm² (20 ASF) atthe cathode and 8-11 mA/cm² (7-10 ASF) at each anode.

An ion exchange resin was prewashed with at least 10 bed volumes ofdeionized water and placed in a glass column. The glass column had adiameter of approximately 1 inch and contained approximately 77.0 cubiccentimeters (4.7 cubic inches) of the ion exchange resin. Theelectrolytic solution was continuously circulated through the glasscolumn by a magnetically coupled 1/250 HP Iwaki pump during theelectrorefining process at a flow rate between 100 and 500 mL perminute.

The tin was electrorefined for three days, and then harvested from thecathode. The harvested tin was rinsed for five minutes with deionizedwater having a purity of 5 megaohms per centimeter. The electrorefinedtin was then dried for 15 minutes at 150° C., and cast at 300° C.-350°C. Three crops were harvested for each example. A sample was taken fromeach crop, and analyzed by an Alpha Sciences 1950 alpha counter in themanner described in JEDEC standard JESD221 and a Varian Vista Proinductive coupled plasma atomic emission spectroscopy (ICP-AES) fortrace elements.

Control

The Control did not include an ion exchange resin in the electrorefiningprocess. A sulfuric acid electrolyte was formed by mixing 3% sulfuricacid by volume with deionized water. Tin from the anodes waselectrolytically dissolved from high purity tin anodes in the sulfuricacid electrolyte to form a 15 g/L solution. Technistan Antioxidant (anantioxidant) was added at a volume percent of 1% by volume of the totalelectrolytic solution and Technistan TP-5000 additive (an organic grainrefiner) was added at a volume percent of 4% by volume of the totalelectrolytic solution. The electrolytic solution had a pH of less thanabout 1 (calculated pH).

Electrolysis was performed at 20° C. using a cathode current density of22 mA/cm² (20 ASF). The cathodes were harvested after 72 hours. The tinwas cast. The casts were analyzed by the Alpha Sciences 1950 alphacounter (in the manner described in JEDEC standard JESD221) and theVarian Vista Pro ICP-AES. The mean alpha particle emissions (incounts/hour/cm²) and standard deviation (“SD”) based on three samplesare shown in Table 4 as measured immediately after casting (“refinedalpha”) and after storage for at least 90 days (“alpha after 90 days”).

TABLE 4 Refined tin sample data Refined alpha Alpha after 90 daysPercent Percent Starting alpha reduc- reduc- Mean SD Mean SD tion MeanSD tion 0.0119 0.0080 0.0005 0.008 96% 0.0068 0.00046 15%

The electrorefining process of the Control, which did not include an ionexchange resin, reduced the alpha particle emissions by 96% immediatelyfollowing the refining process. However, the alpha particle emissions oralpha flux increased after 90 days, resulting in an alpha reduction ofonly 15%.

Samples 1-3

Samples 1-3 included an ion exchange resin in the electrorefiningprocess. An electrolytic solution containing sulfuric acid, deionizedwater, tin, Technistan Antioxidant and Technistan TP-5000 was preparedas described above for the Control.

Electrolysis was performed at 20° C. using a cathodic current density of22 mA/cm² (20 ASF). Electrolytic solution from the main tank was pumpedthrough the glass column which contained the designated ion exchangeresin at the designated flow rate. The ion exchange resin and flow ratesare presented in Table 5.

TABLE 5 Electrorefining process information Ion Exchange Resin Flow RateSample 1 Monophos 450 ml/min Sample 2 Lewatit MonoPlus 320 ml/min TP 260Sample 3 Reillex HPQ 210 ml/min Polymer

The cathodes were harvested after 72 hours from the start of theelectrorefining process. The electrorefined tin was cast, and the castswere analyzed by the Alpha Sciences 1950 alpha counter (in the mannerdescribed in JEDEC standard JESD221) and the Varian Vista Pro ICP-AES.The mean alpha particle emissions (counts/hour/cm²) and standarddeviation (“SD”) for three samples as measured immediately after casting(“refined alpha”) and at least 90 days after casting (“alpha after 90days”) are shown in Table 6. The percent reduction (“% reduct.”) of meanalpha particle emissions based on the starting alpha particle emissionsis also shown.

TABLE 6 Refined tin alpha emissivity data Refined alpha Alpha after 90days Starting alpha % % Sample Mean SD Mean SD Reduct. Mean SD Reduct. 10.0043 0.0018 0.0002 0 95 0.0005 0.0005 88 2 0.0054 0.0016 0.0001 0.000199 0.0007 0.0006 88 3 0.0052 0.0020 0.0001 0.0001 98 0.0005 0.0002 90

Alpha particle emissions of Samples 1-3 immediately after refining andcasting were similar to that of the control. Ninety (90) days aftercasting, the alpha particle emissions of Samples 1-3 were significantlyreduced compared to the control.

The lead content of the samples were analyzed before and afterelectrorefining by Varian Vista Pro ICP-AES. The lead content forSamples 1-3 are provided in Table 7.

TABLE 7 Refined tin lead content data Refined Pb Lot Starting Pb (ppm)(ppm) Sample 1 1 5 5 2 5 5 3 4 5 Sample 2 1 4 5 2 4 4 3 4 4 Sample 3 1 44 2 4 4 3 4 4

Electrorefining did not significantly change the lead content in Samples1-3. Further, any measured change in lead content is within theexperimental margin of error.

Samples 4-20

Samples 4-20 included an ion exchange resin in the electrorefiningprocess. An electrolytic solution containing sulfuric acid, deionizedwater, tin, Technistan antioxidant and Technistan TP-5000 was preparedas described above for the Control.

Electrolysis was performed at 20° C. using a cathodic current density of22 mA/cm² (20 ASF). Electrolytic solution from the main tank was pumpedthrough the glass column which contained the designated ion exchangeresin at the designated flow rate. The ion exchange resin, flow rates(mL/min), alpha particle emissions (counts/hour/cm²), including mean andstandard deviation (“SD”) are presented in Table 8.

TABLE 8 Refined tin sample data Refined alpha Ion Percent Sam- ExchangeFlow Starting alpha reduc- ple Resin Rate Mean SD Mean SD tion 4 DowexG- 280 0.003 0.0005 0.0004 0.0001 87% 26 5 Dowex 227 0.003 0.0005 0.00060.0003 79% Optipore L493 6 Dowex 3.8 0.003 0.0005 0.0006 0.0004 80%MAC-3 7 Amberlite 3.6 0.003 0.0005 0.0003 0.0001 90% IRC-747 8 Diphonix355 0.003 0.0005 0.0009 0.0015 69% resin 9 Amberlyst 240 0.003 0.00050.0017 0.0013 44% A-26 10 Dowex 260 0.003 0.0005 0.0003 0.0003 91% PSR-211 Amberlyst 400 0.003 0.0005 0.0006 0.0006 79% 15WET 12 Lewatit 2350.00413 0.00196 0.0000 0.0000 100% TP-260 13 XZ 285 0.00413 0.001960.0005 0.0005 88% 91419.00 resin 14 Lewatit 235 0.00413 0.00196 0.00010.0001 97% TP-207 15 XUS 320 0.00413 0.00196 0.0001 0.0001 98% 43568resin 16 Amberlite 400 0.00640 0.0005 0.0005 0.0005 93% PWA 5 17 Dowex240 0.00640 0.0005 0.0005 0.0004 92% 21K XLT 18 Dowex 210 0.00640 0.00010.0004 0.0001 94% G-26 19 Amberlite 430 0.00640 0.0001 0.0008 0.0001 88%IRC 747 20 Reillex 250 0.00640 0.0001 0.0009 0.0001 85% HP

The alpha particle emissions were reduced the greatest amount in Sample12 (100%), which included Lewatit TP-260 ion exchange resin and wasreduced the least in Sample 9 (44%).

The lead content of the samples were analyzed before (e.g.,pre-refining) and after (e.g., post-refining) electrorefining by theVarian Vista Pro ICP-AES. Three samples, or lots, were analyzed for eachresin tested. The lead content for Samples 4-20 are provided in Table 9.

TABLE 9 Refined tin sample lead content data Starting Refined Sample LotPb (ppm) Pb (ppm) 4 1 5 6 2 5 5 3 5 4 5 1 5 5 2 5 6 3 5 4 6 1 5 5 2 5 53 5 5 7 1 5 6 2 5 5 3 5 4 8 1 5 5 2 5 5 3 5 4 9 1 5 5 2 5 4 3 5 4 10 1 55 2 5 5 3 5 4 11 1 5 5 2 5 5 3 5 4 12 1 5 6 2 5 5 3 4 5 13 1 5 5 2 5 5 34 4 14 1 5 5 2 5 5 3 4 5 15 1 5 6 2 5 5 3 4 5 16 1 4 5 2 4 5 3 4 5 17 14 6 2 4 5 3 4 5 18 1 4 5 2 4 5 3 4 5 19 1 4 5 2 4 5 3 4 5 20 1 4 5 2 4 53 4 3

Electrorefining did not significantly change the lead content in Samples4-20.

Example 2 Adjustment of Tin Concentration and Current Density in anElectrorefining Process

The effects of tin concentration and current density were investigatedin Samples 21-25. Electrolytic solutions containing sulfuric acid,deionized water, tin, Technistan Antioxidant and Technistan TP-5000 wereprepared as described above for the Control.

During the electrodeposition process, the electrolytic solution from themain tank was pumped through the glass column containing LewatitMonoPlus TP 260 ion exchange resin. The tin was deposited at 20° C. andonto a cathode having an active area of 72 square inches. The tinconcentration of the electrolytic solution, the cathodic current in ampsand the cathodic current density in ASF for each sample is provided inTable 10.

TABLE 10 Electrorefining process information Tin Current concentrationCurrent density Sample (g/L) (Amps) (ASF) 21 20 5 10 22 40 15 30 23 2025 50 24 60 5 10 25 60 25 50

Before the electrorefining process, the input or pre-refined tin hadalpha particle emissions of 0.048 counts/hour/cm². The post-refinedalpha particle emissions and elapsed time between refining and themeasurement of alpha particle emissions are shown below in Table 11. Thealpha particle emissions were measured at multiple elapsed times forselect samples.

Table 11 also includes percent reduction and the reduction factor of themeasured alpha particle emissions as compared to the input orpre-refined alpha particle emissions. The percent reduction wascalculated by the difference between the pre-refined and post-refinedalpha particle emissions divided by the pre-refined alpha particleemissions. The reduction factor was calculated by the pre-refined alphaparticle emissions divided by the post-refined alpha particle emissions.

TABLE 11 Refined tin sample alpha emissivity data Alpha particle Elapseemissions time Percent Reduction Sample Lot (counts/hr/cm2) (days)reduction factor 21 1 0.02 26 58% 2.4 2 0.0138 23 71% 3.5 3 0.009 21 81%5.3 3 0.021 47 56% 2.3 22 1 0.0137 13 71% 3.5 1 0.026 39 46% 1.8 2 0.0129 75% 4.0 3 0.0103 6 79% 4.7 23 1 0.0136 19 72% 3.5 2 0.0094 16 80% 5.13 0.0095 10 80% 5.1 3 0.0301 39 37.3%   1.6 24 1 0.013 35 73% 3.7 10.024 54 50.0%   2.0 2 0.0093 27 81% 5.2 3 0.0058 20 88% 8.3 25 1 0.002914 94% 16.6 1 0.0088 34 81.7%   5.5 2 0.0035 14 93% 13.7 3 0.0016 13 97%30.0 3 0.0093 34 81% 5.2

Sample 21, which had the lowest tin concentration and the lowest currentdensity, provided the least reduction in alpha particle emissions.Sample 25, which had the highest tin concentration and the highestcurrent density, provided the greatest reduction in alpha particleemissions.

A plot of the alpha particle emissions over time for each sample isprovided in FIG. 2. A linear trend line was fit to each data set, andthe equations are presented in FIG. 2. The linear trend line for Sample22 had a slope of 0.0005, Sample 23 had a slope of 0.0008, Sample 24 hada slope of 0.0005 and Sample 25 had a slope of 0.0003. A linear trendline could not be fit to the data for Sample 21.

Example 3 Determination of Maximum Alpha Emissions in Refined TinSamples

The present method was used to assess the maximum potential alphaemissions in eight refined tin samples. The tin samples were refinedaccording to the method described herein. Test samples of the refinedtin samples were obtained by cutting an approximately 1 kilogram samplefrom an ingot and rolling the sample to a thickness of 1 millimeter. Thetest samples were heated at 200° C. for six hours, and the alphaparticle emissions of the test samples were measured using an XIA1800-UltraLo gas ionization chamber available from XIA L.L.C. ofHayward, Calif. The measured alpha particle emissions and elapsed timesbetween refining and the measurement of alpha particle emissions areshown below in Table 12.

TABLE 12 Refined tin sample data Elapsed time (t) between Maximumrefining and ²¹⁰Pb alpha Alpha particle measurement of concentrationparticle emissions alpha particle at time = 0 emission (alpha flux)emissions (atoms/cm²) (equation Sample (counts/hr/cm²) (days) (equation(2)) (3)) 26 0.002 89 66 0.0056 27 0.0045 258 74 0.0063 28 0.0016 113 440.0037 29 0.004 272 64 0.0055 30 0.0016 211 29 0.0025 31 0.0009 32 720.0061 32 0.025 553 324 0.0276 33 0.0195 523 255 0.0217

From the measured alpha particle emission and the elapsed time (t)between refining and the measurement of alpha particle emission, theconcentration of ²¹⁰Pb at (t)=0 can be calculated from equation (3)above.

For example, the alpha particle emission of Sample 26 was measured at0.002 counts/hr/cm² at 89 days from refining. Based on equation (3)above, the number of ²¹⁰Pb atoms per cm² ([²¹⁰ Pb]₀) needed to generatethe measured ²¹⁰Po activity, i.e., measured alpha particle emission, wascalculated to be 66. Using equation (4) above, the activity or predictedalpha particle emission of ²¹⁰Po at (t)=828 days was calculated as0.0056 counts/hr/cm².

In Sample 32, the alpha particle emission was measured at 0.025counts/hr/cm² at 523 days from refining. The value of [²¹⁰Pb]₀ wascalculated based on equation (3) to be 255 atoms/cm², and the maximumalpha particle emission was calculated based on equation (4) as 0.0217counts/hr/cm².

As may be seen from Samples 26 and 32, the difference between themeasured alpha particle emission and the calculated maximum alphaparticle emission decreases as time (t) approaches 828 days, with thegreater difference for Sample 26 attributable to the alpha particleemission measurement being obtained early in the secular equilibriumcycle (e.g., less time had elapsed from the secular equilibriumdisruption event) before secular equilibrium could be re-establishedafter refining.

Example 3 Determination of Time Required to Diffuse the Target DecayIsotope

The time required to diffuse the target decay isotope in a tin samplewas investigated. Tin samples were refined according to the methoddisclosed herein. A test sample of the refined tin sample was obtainedby cutting a sample from an ingot and rolling the sample to a thicknessof 0.45 millimeter. The test sample was heated at 200 C for one hour,and the alpha particle emissions of the test samples were measured usingan XIA 1800-UltraLo gas ionization chamber available from XIA L.L.C. ofHayward, Calif. Measurement of the alpha particle emissions requiredabout 24 hours, after which the sample was heated for one hour at 200°C. and then measured for alpha particle emissions. This process (e.g.,heat for one hour followed by measurement of alpha particle emissions)was repeated for a total of five heat/measurement cycles. The measuredalpha particle emissions and the total hours the sample was heated at200° C. are shown below in Table 13.

TABLE 13 Refined tin sample data Total Alpha particle hour(s) emissionssample (alpha flux) heated (counts/hr/cm²) 0 0.017 1 0.025 2 0.024 30.027 4 0.025 5 0.026

As can be seen from Table 13, the activity or alpha flux of the sampleincreased from 0.017 counts/hr/cm² to 0.025 counts/hr/cm² after one hourat 200 C. That is, the activity or alpha flux of the tin sampleincreased more than 50% after one hour at 200° C. As further shown inTable 13, there was no significant change in the activity or alpha fluxof the sample when heated for more than one hour at 200° C., suggestingthat one hour at 200° C. was sufficient to achieve a substantiallyuniform concentration of the target decay isotopes throughout thesample.

Without further elaboration, it is believed that one skilled in the artcan, using the preceding description, utilize the present invention toits fullest extent. The preceding preferred specific embodiments are,therefore, to be construed as merely illustrative, and not limitative ofthe remainder of the disclosure in any way whatsoever.

In the foregoing, all temperatures are set forth uncorrected in degreesCelsius and, all parts and percentages are by weight, unless otherwiseindicated.

From the foregoing description, one skilled in the art can easilyascertain the essential characteristics of this invention and, withoutdeparting from the spirit and scope thereof, can make various changesand modifications of the invention to adapt it to various usages andconditions.

What is claimed is:
 1. A method for purifying tin, the methodcomprising: exposing an electrolytic solution comprising tin to an ionexchange resin; and depositing electrorefined tin from the electrolyticsolution.
 2. The method of claim 1, wherein the electrorefined tin hasalpha particle emissions of less than about 0.01 counts/hour/cm².
 3. Themethod of claim 1, wherein the exposing and depositing steps occur atleast partially concurrently.
 4. The method of claim 1, wherein theelectrorefined tin has alpha particle emissions of less than about 0.001counts/hour/cm².
 5. The method of claim 1, wherein the ion exchangeresin comprises functionalized carboxylic acid from the phosphonic acidsgroup.
 6. The method of claim 1, wherein the ion exchange resincomprises phosphomethylated functional groups.
 7. The method of claim 1,wherein the ion exchange resin comprises amino methyl phosphonic acidfunctional groups.
 8. The method of claim 1, wherein the ion exchangeresin comprises poly(4-vinyl-pyridine) functional groups.
 9. The methodof claim 1, wherein a lead content of the electrorefined tin is reducedby less than about 1% as compared to tin prior to the exposing step. 10.The method of claim 1, wherein at least 90 days after the depositionstep, the electrorefined tin has an alpha particle emissions of lessthan about 0.01 counts/hour/cm².
 11. The method of claim 1 and furthercomprising: detecting alpha particle emissions from a sample of thedeposited electrorefined tin; determining a concentration of a targetparent isotope in the sample of the deposited electrorefined tin fromthe alpha particle emissions detected in said detecting step and a timewhich has elapsed between said detecting step and said exposing anddepositing steps; and determining an alpha emission potential of atarget decay isotope of the target parent isotope from the determinedconcentration of the target parent isotope and the half-life of thetarget parent isotope.
 12. The method of claim 1, further comprising theadditional steps, prior to said detecting steps, of: obtaining a sampleof the deposited electrorefined tin; and heating the sample to diffuseatoms of the target decay isotope within the sample until a uniformconcentration of atoms of the target decay isotope is obtainedthroughout the sample.
 13. The method of claim 1, wherein theelectrolytic solution has a tin concentration from about 60 g/L to about180 g/L.
 14. The method of claim 1, wherein the electrolytic solutionincludes at least one additive.
 15. The method of claim 14, wherein theadditive includes at least one member selected from the group consistingof antioxidants and grain refiners.
 16. The method of claim 14, whereinthe additive is present in an amount of at least about 0.5% by volume ofthe electrolytic solution.
 17. The method of claim 1 wherein exposingthe electrolytic solution to an ion exchange resin further comprisespassing the electrolytic solution from a first container through the ionexchange resin in a second container and returning the electrolyticsolution to the first container.
 18. The method of claim 1, wherein theelectrolytic solution has pH of about 1 or less.
 19. The method of claim1, wherein the electrorefined tin has a lead concentration of at leastabout 1 part per million (ppm).
 20. The method of claim 1, wherein atleast 90 days after the step of depositing, the electrorefined tin hasan alpha particle emissions that is at least 75% less than the alphaparticle emissions of the tin prior to the exposing and depositingsteps.